1. Field of the Disclosure
The present disclosure relates to printed circuit boards, and more particularly to a printed circuit board on which differential transmission lines are disposed.
2. Description of Related Art
Differential signal transmission is less prone to generate electromagnetic radiation than conventional signal transmission, and is immune to noise. Therefore, differential signal transmission is beginning to be used more and more for high-speed signal transmission in various electronic devices. Referring to FIG. 4 and FIG. 5, a printed circuit board 101 having a differential transmission line 102c is illustrated. The differential transmission line 102c includes two signal conductors 102a and 102b, which are formed parallel to each other on a top face of the printed circuit board 101. A ground conductor layer 105 is formed on a bottom face of the printed circuit board 101. High frequency signals are supplied to the two signal conductors 102a and 102b, so that they function together as parts of the differential transmission line 102c. 
The signal conductor 102a and the ground conductor layer 105 constitute a first transmission line. The signal conductor 102b and the ground conductor layer 105 constitute a second transmission line. The differential transmission line 102c includes the first and second transmission lines.
The shorter the distance between the signal conductor 102a and the signal conductor 102b, the stronger the coupling which occurs between the two transmission lines of the differential transmission line 102c. Therefore in order to strengthen the coupling between the two transmission lines of the differential transmission line 102c, the gap between the signal conductor 102a and the signal conductor 102b should be reduced.
Furthermore, when the two signal conductors 102a and 102b are asymmetrically disposed on the printed circuit board 101, electromagnetic radiation generated by the differential transmission line 102c is significantly increased. Therefore when designing the differential transmission line 102c, it is best to maintain symmetry between the two signal conductors 102a and 102b along their entire lengths.
However, various complex designing requirements of the printed circuit board 101 impose limits on reducing the gap between the two signal conductors 102a and 102b, and on maintaining the symmetry between the two signal conductors 102a and 102b. 